The present invention relates generally to hermetic compressors of the type having a hermetic housing, wherein a hermetic terminal is provided for carrying electric current into the housing and, more particularly, to such a terminal that will maintain the integrity of its hermetic seal even under overcurrent conditions.
Typically, a hermetic terminal is installed in a hole formed in the housing of a hermetic compressor so that current may be carried to the compressor motor from an external source of power. The terminal comprises a body member welded or otherwise secured to the compressor outer housing and a plurality of conductor pins extending through the body member. In order to seal and electrically insulate the conductor pins relative to the body member, a glass-to-metal seal is employed, having an epoxy and/or silicone rubber overcoating. Both the inner and outer ends of the conductive pins may be provided with conductor tabs so as to facilitate connection to the external current source and to the compressor motor.
A problem associated with prior art hermetic terminals is that, in response to unexpected abnormally high overcurrent conditions, e.g., due to a ground fault or a short circuit, the conductor pins may heat up to the point of melting the conductor pins themselves or the surrounding glass-to-metal seals, thereby resulting in failure of the hermetic terminal.
One method for preventing occurrence of the aforementioned failure mode of a hermetic terminal is to use a fusible link within the conductive path of each conductor pin. In general, the fusible link portion has a smaller cross-sectional area than the remaining portions of the conductor pin, and is designed to fuse first upon the occurrence of an overcurrent condition. When an overcurrent condition begins to occur, the fusible link will fuse and terminate the supply of electric current to the compressor motor before the conductor and/or glass-to-metal seal are destroyed by excessive temperatures.
In one prior art hermetic terminal, a pin assembly includes a fusible link attached to and disposed intermediate two pin parts. Specifically, opposite axial ends of the fusible link are received in respective axial bores in the ends of the pin parts. The ends are then crimped to create electrical contact between the pin parts and the fusible link. It is generally recognized that a crimped attachment provides a high resistance and unreliable electrical connection.
In addition to problems of insufficient electrical contact between the fusible link and the pin parts, some terminal assemblies do not provide a reservoir into which a melted or vaporized fusible link may migrate in the event of an overcurrent condition. Such assemblies risk the possibility of incomplete electrical separation between the pin parts and the fusible link.
In U.S. Pat. No. 4,830,630, a hermetic terminal includes pin assemblies each having a fuse link surrounded by a multipiece protective capsule defining an expansion cavity. In order to exhaust the built-up pressure in the capsule and, at the same time, maintain the integrity of the seal between the conductor pin assemblies and body member of the hermetic terminal, the protective capsule is designed to rupture upon rapid vaporization of the fuse material within the expansion cavity. In such an assembly, an outer sealing member fractures upon rupture of the protective capsule, thereby causing the pin parts to separate from the remainder of the terminal. A disadvantage of this hermetic terminal design is the existence of many component parts, including the requirement of a fracturing sealing member surrounding the protective capsule on an extended inner or outer side of the terminal body.